Instead use container_of to retrieve the s3c_hsudc from the
struct usb_gadget pointer.

[ balbi@ti.com : changed verbose container_of() into
	an already provided helper 'to_hsudc()' ]
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>
Signed-off-by: NFelipe Balbi <balbi@ti.com>